Method for measuring chemical levels using pH shift

ABSTRACT

A method of measuring the free chlorine level in a solution of chlorinated pool/spa water comprises a first sample of said solution having a first selected pH and a second sample of said solution having a second selected pH and determining first and second ultraviolet light (UV) transmissivity values for each of the first and second samples. The first and second transmissivity values are then used to determine the free chlorine level.

FIELD OF INVENTION

The subject invention relates to halogen detection in fluid solutionsand more particularly to a chlorine concentration detection method foranalyzing chlorine concentration in spa or pool water wherein the systememploys UV spectral analysis of a solution at two different pH levels.

RELATED ART

In the prior art, various devices for measuring chlorine (hypochlorousacid) concentration in water are known. Such devices include OxidationReduction Potential (ORP) sensors, amperometric sensors and Palin N,N-Diethyl-P-Phenylenediamine (DPD) testing apparatus.

SUMMARY

The following is a summary of various aspects and advantages realizableaccording to various embodiments of the invention. It is provided as anintroduction to assist those skilled in the art to more rapidlyassimilate the detailed discussion which ensues and does not and is notintended in any way to limit the scope of the claims which are appendedhereto in order to particularly point out the invention.

According to various embodiments, a method for measuring the freechlorine level in a solution of chlorinated pool/spa water is providedwherein a pH adjusting device produces a first sample of the solutionhaving a first selected pH and a second sample of the solution having asecond selected pH. A spectral analyzer then determines first and secondultraviolet light (UV) transmissivity values for each of said first andsecond solution samples. The results of spectral analysis may then besupplied to an electronic processor for determining the free chlorinelevel using the first and second transmissivity values.

In one embodiment, the pH adjusting device may comprise first and secondcell solutions, each having a respective electrode positioned therein. Aswitching mechanism may then be employed to switch the polarity of avoltage applied to the electrodes such that a voltage of a firstpolarity is applied to a first of the successive samples and a voltageof a second polarity is applied to a second of the successive samples,thereby developing samples having two different pH values.

In another embodiment, the pH adjusting device may comprise a singlecoaxial cell including first and second coaxial fluid flow chambers anda pair of electrodes. Successive solution samples are supplied to thecell and their pH appropriately altered.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned features and objects of the present disclosure willbecome more apparent with reference to the following description takenin conjunction with the accompanying drawings wherein like referencenumerals denote like elements and in which:

FIG. 1 is a graphical illustration of an embodiment of a percentage ofultraviolet light transmitted through a water solution containing thechlorous ion at various pH levels at a temperature of 23 degreescentigrade at a concentration of chlorine of 3 parts per million;

FIG. 2 is a block diagram of an embodiment of the chlorine measurementsystem of the present disclosure;

FIG. 3 is a block diagram of an embodiment of a spectrum analyzer thatmay be used to measure a percentage of ultraviolet light transmittedthrough water according to the present disclosure;

FIG. 4 is a block diagram of an embodiment of the chorine measurementsystem of the present disclosure;

FIG. 5 is a block diagram of an embodiment of the chlorine measurementsystem of the present disclosure;

FIG. 6 is a perspective view of an embodiment of an integratedelectrolysis cell and optical cell;

FIG. 7 is a cross sectional view of an electrolysis cell of an apparatusfor measuring chlorine levels of the present disclosure;

FIG. 8 is a perspective view of an embodiment of an outer cylindricaltube of an electrolysis cell;

FIG. 9 is a perspective view of an embodiment of an outer graphiteelectrode of an electrolysis cell;

FIG. 10 is a perspective view of an embodiment of a ceramic separatortube of an electrolysis cell;

FIG. 11 is a perspective view of an embodiment of an inner graphiteelectrode of an electrolysis cell;

FIG. 12 is a top (interior) view of an embodiment of an electrolysiscell input end cap;

FIG. 13 is a side view of an embodiment of an electrolysis cell inputend cap;

FIG. 14 is a side view of an embodiment of an electrolysis cell inputend cap;

FIG. 15 is a top (interior) view of an embodiment of an electrolysiscell output end cap;

FIG. 16 is a side view of an embodiment of an electrolysis cell outputend cap;

FIG. 17 is a side view of an embodiment of an electrolysis cell outputend cap;

FIG. 18 is a top (interior) view of an embodiment of an optical cell endcap;

FIG. 19 is a side view of an embodiment of an optical cell end cap;

FIG. 20 is a side view of an embodiment of an optical cell end cap;

FIG. 21 is a perspective view of an embodiment of an optical covermember for holding a quartz window in place in an optical cell;

FIG. 22 is a perspective view of an embodiment of an end plate thatattaches to an end cap of an optical cell;

FIG. 23 is a block diagram of an embodiment of electronic controlcircuitry that controls an apparatus for measuring chlorine dioxidelevels; and

FIG. 24 is a block diagram of an embodiment a plumbing system employingthe apparatus for measuring chlorine levels.

DETAILED DESCRIPTION

In the following detailed description of embodiments of the invention,reference is made to the accompanying drawings in which like referencesindicate similar elements, and in which is shown by way of illustrationspecific embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that logical, mechanical,electrical, functional, and other changes may be made without departingfrom the scope of the present invention. The following detaileddescription is, therefore, not to be taken in a limiting sense, and thescope of the present invention is defined only by the appended claims.As used in the present disclosure, the term “or” shall be understood tobe defined as a logical disjunction and shall not indicate an exclusivedisjunction unless expressly indicated as such or notated as “xor.”

The methods of the present disclosure relate to chemical, particularlyhalogen, detection in fluid solutions and may be particularly adaptedfor use in pools and spas, but also in potable water solutions and othersimilar applications wherein halogens are added. As chlorine is a commonadditive to pool and spa water, the present disclosure is describedusing chlorine as an exemplary chemical. Bromine, iodine, and the otherhalogens are expressly contemplated for use in the present system.Indeed, the apparatuses and methods of the present disclosure areapplicable to nearly any chemical in solution, which allowsdetermination of the concentration of the chemical. Artisans willrecognize the modifications, such as absorption wavelength appropriateon a case by case basis without undue experimentation.

FIG. 1 illustrates a graph of the percentage of ultraviolet (UV) lighttransmitted through a water solution containing the chlorous ion (OCl⁻)for various pH levels at a temperature of 23 degrees centigrade and aconcentration of free chlorine of 3 parts per million (ppm). From FIG.1, it may be seen that the UV transmission percentage changes as the pHshifts, e.g. at the wavelength of 293 nanometers (nm). Other wavelengthsof electromagnetic radiation may also be employed depending on thechemical being measured and a useful or optimal absorptioncharacteristic of particular chemical. For example, wavelengths in theinfrared or visible spectrum are expressly contemplated.

A chlorine measurement system which takes advantage of the pH shiftphenomenon illustrated in connection with FIG. 1 is shown in FIG. 2.First cell and second cell 15, 17 respectively, both electrochemicalhalf-cells are provided, each containing a respective half-cellsolution. Each half-cell solution may comprise, for example, a sample ofchlorinated pool or spa water containing hydrochlorous acid and thechlorous ion (HOCl/OCl⁻).

The two half-cell solutions are separated by separator 14, e.g. by asalt-bridge or a semi-permeable membrane or permeable membrane. Apositive electrode 23 is placed in the half-cell solution in first cell15, while a negative electrode 25 is placed in the half-cell solution insecond cell 17. In operation, negative and positive voltages are appliedto the positive electrode 23 and the negative electrode 25,respectively. Application of these voltages causes acidity of thehalf-cell solution of the first cell 15 to increase, while acidity ofthe half-cell solution of the second cell 17 decreases.

To compare the respective transmission percentages of the respectivesolutions in the first cell 15 and second cell 17, respective samplesare supplied in succession to an analyzer 31. For example, a firstsample from the first cell 15 may be supplied via a conduit 27 through a“T” valve 30 to the analyzer 31. After analysis of that first sample, asecond sample is provided from the second cell 17 via a conduit 26through a “T” valve 28 to the analyzer 31. According to embodiments, theanalyzer 31 is a spectrum analyzer and determines the respectivetransmission percentages at a selected frequency, for example, 293 nm,and then uses the respective transmission percentages to calculate thelevel of the OCl⁻ ion. Using such a differential measurement techniquehas the advantage that other contaminants in the solution whose UVabsorption does not change with pH will have no effect on the resultingOCl⁻ measurement and therefore will not skew that measurement.

To determine the chlorine level from the difference measurement, a tablemay be provided that comprises the spectral difference corresponding toeach of a range of chlorous ion levels at a selected wavelength. Such atable may be created by measuring the spectral difference of a range ofsamples, each of whose chlorous ion level is known, for example.

In operation of the system of FIG. 2, when one cell 15, 17 has itshalf-cell solution directed to the analyzer 31, the other cell's 17, 15half-cell solution must be directed back to its source, for example aspa or a pool. Both cells 15, 17 need to have flowing half-cellsolutions for the system to operate as intended. Bypass return conduits21, 29 provide an outlet to the source of the half cell solution thatbypasses analyzer 31. For example, when “T” valve 30 is open anddirecting half-cell solution of first cell 15 to analyzer 31, then “T”valve 28 is open to direct the half-cell solution of second cell 17 backto the source of the solution through bypass conduit 29. Otherwise, abuild up of reactants will occur in the static solution, causing a buildup of acid or base and excessive breakdown of HOCl.

FIG. 3 depicts an illustrative analyser 31. The analyzer 31 includes aUV source 39, a filter 37, a cuvette 35, a detector 41, and analysiselectronics 43. The UV source 39 and filter 37 combination provide a 293nm wavelength, which passes through the solution sample contained incuvette 35 to the detector 41, which may be a conventional UV detector,according to embodiments. The output of the detector 41 is applied toanalysis electronics 43, which performs analog to digital (A-D)conversion and digital comparison or subtraction of the respectivetransmission readouts from first cell 15 and second cell 17. Theanalysis electronics 43 then perform a table look-up operation todetermine the chlorous ion level. Analysis electronics 43 may comprise asuitable A-D converter and a microcontroller and/or computer.

FIG. 4 illustrates an alternative embodiment wherein single cell 55 isused rather than the dual cell embodiment of FIG. 2. In this embodiment,a first sample of half-cell solution is subjected to a positive chargeand then analyzed. Thereafter, a second sample of half-cell solution isexposed to a negative charge and that sample is then passed to theanalyzer 31 through outlet conduit 62. A switch 58 is depicted as beingused to switch between a positive electrode 60 and a negative electrode61. A valve 59 is shown to indicate the switching of successivehalf-cell solution samples into the dual-purpose cell 55 through inletconduit 57.

FIG. 5 shows a more detailed implementation of an embodiment similar toFIG. 4. As in FIG. 2, first cell 315 and second cell 317 are separatedby a separator 314. Half-cell solution is supplied via a conduit 319 tofirst cell 315 and second cell 317 and exits via a conduit 321. Aswitchable voltage source V 323, 325 is arranged to apply a voltage of afirst polarity to a first half-cell solution sample. A sample of thefirst-half-cell solution at a first pH value in the second cell 317 isthen supplied to an analyzer 331 for analysis via conduit 329.Thereafter, the first half-cell solution sample is discharged via theconduit 321, and a second half-cell solution sample is loaded into firstcell 315 and second cell 317 via the conduit 319. The polarity of thevoltage source V is then reversed to develop a second half-cell solutionsample having a second pH value for supply to the analyzer 331.

It may be observed that too much charge per unit mass can cause the pHat the negative electrode 61 to increase beyond 8.2-8.3, which leads toa highly absorbing spectra solution. Too high a voltage and drivecurrent will therefore cause HOCl/OCl⁻ to be electrolyzed and lost fromthe solution and also will cause chlorine to be emitted as well asoxygen.

FIG. 6 illustrates a co-axial embodiment of an electrolysis cell 103 andan optical cell 105. The co-axial electrolysis cell 103 is disposedbelow the optical cell 105 and both are disposed at an acute angle θ, tothe horizontal. The angle θ may be, for example, between 10° and 80°, toallow gas bubbles to be flushed out of an exit port 95.

The electrolysis cell 103 includes a central cylindrical tube 106 havingfirst and electrolysis cell input and output end caps 108, 107, at eachof its respective ends. Tubing 90 from a high pressure side is connectedto the single inline port of die electrolysis cell input end cap 108.The tubing 90 may be, for example, clear PVC tubing. In the embodimentof FIG. 6, pH modulation is obtained by controlling the voltage on anelectrode supply cable 91.

The electrolysis cell output end cap 107 has two outputs, 92, 93. Theoutput 93 may be, for example, clear PVC tubing, which returns half-cellsolution to the source. The second output 92 may comprise black PVCtubing extending from the outermost outlet of the end cap 108. Thesecond output 92 is supplied to the input entry inlet 94 of the opticalcell 105.

The optical cell 105 comprises a central cylindrical tube 212, ontowhich are mounted respective end caps 109 and 111. An end plate 210 isattached to each end cap 109, 111. The lower end cap 210 receives a UVsource cable 102, while the upper end cap 210 receives a UV sensor cable104. An exit port 95 is positioned on the topside of the optical cell105. The cylindrical tube 212 is hollow.

FIG. 7 shows a cross-sectional view of the central cylindrical tube 106of the electrolysis cell 103 (FIG. 6). Within the outer cylindrical tube112, several components are concentrically disposed, including a outergraphite electrode 113, a ceramic separator tube 117, and an innergraphite electrode 121. The concentric arrangement of these componentsdefines an outer slow chamber 115 and an inner flow chamber 119. Each ofthese components will now be described in detail. It will be appreciatedthat the embodiment described herein, including the dimensionsdescribed, are merely exemplary embodiments; equivalent structures orvarying the dimensions are expressly contemplated.

FIG. 8 illustrates the outer cylindrical tube 112 of the electrolysiscell 103 in more detail. The outer cylindrical tube 112 may comprise a1¼-inch grey standard PVC pipe, having, for example, an inside diameterof 28.60 mm and a length of 94.5 mm.

The outer graphite electrode 113 is illustrated further in FIG. 9. Theouter graphite electrode 113 may have a length of 94.5 mm and an outsidediameter of, for example, 28.50 mm so that it snugly mates with theouter cylindrical tube 112. The outer graphite electrode 113 furtherincludes a cylindrical plug that fits within large inner cylindricalrecess 123 (see FIG. 12) at an end thereof, around which an electricallyconductive wire is wrapped, so that it is in intimate contact with theouter graphite electrode 113.

The ceramic separator tube 117 is illustrated in further detail in FIG.10. It may have a length of, for example, of 100.50 mm, an insidediameter of 6.50 mm, and an outside diameter of 13.80 mm.

The inner graphite electrode 121 is illustrated in further detail inFIG. 11. It may have a length of 106.50 mm, an inner diameter of 3.5 mm,and an outer diameter of 5.0 mm.

The electrolysis cell input end cap 108 is illustrated in further detailin FIGS. 12 through 14. As shown, the electrolysis cell input end cap108 includes a sidewall 128, which snugly receives the outer cylindricaltube 112 of the electrolysis cell 103. According to embodiments, a largeinner cylindrical recess 123 is shaped and dimensioned to snugly receivethe ceramic separator tube 117 (See FIG. 7). Single input port 131,having an half-cell solution input conduit 132 ending at input well 127,allows the large inner cylindrical recess 123 to be in fluidcommunication with a half-cell solution source. As shown in FIG. 14,large inner cylindrical recess 123 positions ceramic separator tube 117above the plane in which input well 127 opens. Therefore, as half-cellsolution flows into input well 127 via half-cell solution input conduit132, ceramic separator tube 117 divides the flow of half-cell solutionentering into the electrolysis cell 103 into inner flow chamber 139 andouter flow chamber 115.

A small inner cylindrical recess 125 of lesser diameter than the largeinner cylindrical recess 123 is concentrically disposed within the largeinner cylindrical recess 123 and shaped and dimensioned to snuglyreceive the inner graphite electrode 121. Accordingly, each of outercylindrical tube 112, ceramic separator 117, and inner graphiteelectrode 121 are registered in their proper orientation and positionwithin the electrolysis cell 103. Two small apertures 135, 136 arefurther provided to for the electrical leads.

The electrolysis cell output end cap 107 is shown in further detail inFIGS. 15 through 17. The electrolysis cell output end cap 107 includes arecess 141 for snugly receiving the outer cylindrical tube 112. Theelectrolysis cell output end cap 107 also comprises concentric largeouter cylindrical recess 143 and small outer cylindrical recess 145 forreceiving and registering the ceramic separator tube 117 and the innergraphite electrode 121, respectively. Output channels 147, 149 areprovided to provide outlets from the inner flow chamber 119 and outerflow chamber 115 through respective ports 151, 152.

The optical end cap 109 of optical cell 105 is illustrated in FIGS. 18through 20. As shown, the optical end cap 109 is generally cylindricalin shape and has an interior cylindrical wall 161 for mating with theouter cylindrical tube 212. Because of its symmetrical design, the endcap 109 can be used cm both the inlet and outlet ends of the opticalcell 105.

The optical end cap 109 includes a fluid flow port 163 with conduit 165placing the interior of the optical cell 105 into fluid communicationwith exterior components of the present disclosure. A cylindricalopening 175 is provided for mounting quartz window 173. The quartzwindow 173 and a cooperating O-ring seal are held in place via theoptical cover member 179 of FIG. 21.

Referring still to FIG. 21, the optical cover member 179 containscylindrical openings 172, 171 which register with tapped openings 169,170 in the interior of the optical end cap 109. Fastening devices maythen be inserted through the openings 171, 172 and threadably attachedvia tapped openings 169, 170 to securely seal the quarts window 173. Asmay be appreciated, the cylindrical opening 175 extends to the exteriorend surface 185 of the end cap 109.

Turning again to FIGS. 18 to 20, tapped openings 166, 167, 168 are alsoformed into the exterior of the optical end cap 109. These tappedopenings permit attachment of the optical end plate 210 illustrated inFIG. 22. Openings 186, 187, 188 on optical end plate 210 register withthe openings 166, 167, 168 on the optical end cap 109 to permitattachment of the end plate 210 to optical end cap 109.

The end plate 210 further includes a cylindrical opening 181, which atthe input end of the optical cell 105 mounts a UV source 213 (see FIG.23), which is a UV LED according to an embodiment. On the output end ofthe optical cell 105, an optical end plate 210 mounts and retains aphoto detector 215, functioning as a UV detector. Thus, in operation, aUV source 213 retained at the input end of the optical cell 105 directsUV illumination through the solution contained within the centralcylindrical tube 212, which is then detected by the photo detector 215mounted in the output end cap 109. A cylindrical opening 189 facilitatescable strain relief/attachment and passage of suitable electricalcables.

The electronic control circuitry for tire system shown in FIG. 23 isconfigured around processor 203, which may be, for example, an eight-bitmicrocontroller. With respect to the electrolysis cell 103, theprocessor 203 controls the voltage across the cell 103 via voltagecontroller 205, which outputs to a polarity controller 207. The polaritycontroller 207 in an illustrative embodiment may be an H bridge, whichpermits both positive and negative driving polarities to be applied tothe electrolysis cell 103.

The voltage across and current through the electrolysis cell 103 issensed by a current and voltage sensing apparatus 209 and signalsrepresentative of those respective quantities are supplied to theprocessor 203. The voltage and current measurements are measurementsthat allow the processor 203 to detect both normal and abnormaloperation of the system. At a low voltage, the conductivity of thesolution may be determined, which is a parameter related to the level ofsoluble salts in the spa water solution. Abnormally high conductivitymay be used to indicate that the water needs to be replaced with cleanwater. An abnormally low conductivity may indicate that there is nowater present in the sensors, and that the spa is empty, or some otherfault is present that requires intervention to correct. The processor203 may then indicate a fault condition by illuminating a ‘fault’ light.During normal operation, the processor 203 uses the conductivity readingto drive the electrolysis cell 103 with a constant current, by applyinga variable voltage. This ensures that a constant change in pH isobtained, independent to a variable solution conductivity.

With respect to the optical cell 105, the processor 203 controls thecurrent level supplied to the UV source 213 from a constant currentsupply 211. The UV source 213 may be a UVTOP LED as available fromSensor Electronic Technology, Inc., 1195 Atlas Road, Columbia, S.C.29209. The photo detector 215 develops a signal representative of thetransmitted UV light and supplies that signal to an amplifier/analog todigital converter 217 (ADC). The output of the amplifier/ADC 217 is thensupplied to the processor 203 which performs those computationsnecessary to determine the chlorine level.

In one embodiment, the processor 203 takes three UV absorption readingsand uses these values to determine the hypochlorite level. The firstreference reading, R1, is of the spa solution when no pH shift has takenplace, the second, R2, when a positive shift has taken place and thethird, R3, after a negative shift has taken place. The shift readingsare normalized, by dividing them by the first reference reading. Thedifference between the two normalized shift readings is taken, and theanswer logged. The result, X, is a value that is directly proportionalto the free hypochlorite level. The exact level of hypochlorite can bedetermined by calculation, using X to complete the only unknown in alinear equation of the form Y=MX+C, where M and C are constantspreviously found by calibration and Y is the level of tree hypochloritein parts-per-million.X=Log [(R2/R1)−(R3/R1)]  Equation (1)Y=MX+C  Equation (2)

Thus, combining (1) and (2), the equation relating free chlorine tomeasurements is:Y=M(Log [(R2/R1)−(R3/R1)])+C  Equation (3)

If processing power is a limited resource, but sufficient memory isavailable in the processor 203, then an alternative method can be usedto determine the free chlorine level. In this method, an intermediatevalue is found, so as to avoid the logarithmic calculation, which iscomputationally expensive on low cost microprocessors. The value D isobtained by normalizing and differencing the sensor readings. The valueof Y, the level of free hypochlorite, is found by searching down alook-up table of values of D until a matching entry is found at index I.The free chlorine level can then be read out of the table, by examiningits entry located at index I.D=(R2/R1)−(R3/R1)  Equation (4)N(I)=D, to find I  Equation (5)Y=Y(I)

According to an embodiment, the basic commands for the processor 203 arerelatively simple:

a. take a reading (“s” command)

b. take a current reading (“i” command)

c. take a voltage reading (“v” command)

d. set the electrolysis cell drive level (“e, [number]”).

A plumbing schematic for an illustrative embodiment is shown in FIG. 24.As shown, a conventional spa pump, heater, and filter circuit suppliesspa water at a relatively low flow rate to the electrolysis cell 103,which outputs to the optical cell 105. Both the optical cell 105 andelectrolysis cell 103 output to respective flow restrictors, and bothflow restrictors connect to the return line to the spa. A smallrestriction from the filter to the return is used to draw most of thepump pressure, thereby permitting a relatively low flow rate at theentrance to the electrolysis cell 103.

With respect to design considerations, it is desirable to avoid very lowflow rates. It is further desirable to ensure a stable flow of waterrunning through both paths and to further ensure that the entry and exitfluid streams are at the same temperature as the spa.

In operation of the system, a number of pH readings are taken. An “s”command is sent to the electronic controller, and then, over a period of250 milliseconds, the unit takes sixteen readings and returns an answerin the format “s, [number]” to the processor 203.

Next the pH is changed. This change is accomplished by sending a command“e, [number]” to the H bridge, after which the voltage and polarityacross the electrolysis cell 103 are set. UV transmission data is thentaken and examined by the processor 203 to ascertain when the pH of thesolution in the optical cell 105 stabilized, which may take manyminutes.

It is further desirable to confirm that a valid pH change has occurred,either by measuring the pH directly or by calculating it based on aknown flow rate, V drive, I drive, and the water conductivity measuredat a low V drive.

Once valid absorption data has been taken, an “overdrive” method may beused if desired to calculate free chlorine. In this method, readings aretaken at both high and low pH levels, and the difference between the twosensor measurements is determined. The logarithm of this differencevalue may then be taken to find the absorbance, which is proportional tofree chlorine. The exact chlorine level value may be determined by usinga linear formula based on a calibration curve.

While the apparatus and method have been described in terms of what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the disclosure need not be limited to thedisclosed embodiments. It is intended to cover various modifications andsimilar arrangements included within the spirit and scope of the claims,the scope of which should be accorded the broadest interpretation so asto encompass all such modifications and similar structures. The presentdisclosure includes any and all embodiments of the following claims.

1. A method of measuring the level of a substance in a first solution comprising: changing the pH of a first sample of the first solution to a first selected value, thereby producing a first pH adjusted sample; performing a first measurement of electromagnetic radiation transmissivity of the first pH adjusted sample; changing the pH of a second sample of the first solution to a second selected value, thereby producing a second pH adjusted sample; performing a second measurement of the electromagnetic radiation transmissivity of the second pH adjusted sample; and determining the level of the substance using the result of each of the first and second measurements.
 2. The method of claim 1, wherein the transmissivity of each of the first pH adjusted sample and the second pH adjusted sample is determined at the same wavelength of electromagnetic radiation.
 3. The method of claim 2, wherein the wavelength is 293 nanometers.
 4. The method of claim 1, wherein the solution comprises pool or spa tub water.
 5. The method of claim 4, wherein the substance is a halogen.
 6. The method of claim 5, wherein the substance is chlorine or bromine.
 7. The method of claim 1, wherein the solution comprises water that is intended to be potable.
 8. The method of claim 1, wherein the step of determining comprises determining a difference value between the light transmissivity measured by the first measurement and the light transmissivity measured by the second measurement.
 9. The method of claim 8, wherein the step of determining further comprises comparing the difference value to a table comprising a range of substance levels and a pre-determined difference value corresponding to each level in the range.
 10. The method of claim 1, wherein the step of determining comprises using the result of the first and second measurements in a logarithmic calculation.
 11. The method of claim 1, wherein the pH of the first pH adjusted solution is at a first level of acidity and the pH of the second pH adjusted solution is at a second level of acidity.
 12. The method of claim 1, wherein the first pH adjusted solution is produced by applying a first voltage of a first polarity to the first sample.
 13. The method of claim 12, wherein the second pH adjusted solution is produced by applying a second voltage of polarity opposite to the polarity of said first voltage to the second sample.
 14. The method of claim 13, wherein the same liquid containing vessel is used to produce the first and second pH adjusted samples.
 15. A method of measuring the level of a substance in a first solution comprising: applying a first electrical voltage to change the pH of a first sample of the first solution to a first value, thereby producing a first pH adjusted sample; performing a first measurement of electromagnetic radiation transmissivity of the first pH adjusted sample; applying a second electrical voltage to change the pH of a second sample of the first solution to a second value, thereby producing a second pH adjusted sample; performing a second measurement of the electromagnetic radiation transmissivity of the second pH adjusted sample; and determining the level of the substance in the first solution using the result of each of the first and second measurements.
 16. The method of claim 15, wherein the transmissivity of each of the first pH adjusted sample and the second pH adjusted sample is determined at the same wavelength of electromagnetic radiation.
 17. The method of claim 16, wherein the wavelength is 293 nanometers.
 18. The method of claim 15, wherein the solution comprises pool or spa tub water.
 19. The method of claim 18, wherein the substance is a halogen.
 20. The method of claim 19, wherein the substance is chlorine or bromine.
 21. The method of claim 15, wherein said first measurement and second measurement each comprise a measurement of ultraviolet (UV) light transmissivity.
 22. The method of claim 21, wherein said first and second measurements are made at the same UV wavelength.
 23. The method of claim 15, wherein the step of determining comprises determining a difference value between the light transmissivity measured by the first measurement and the light transmissivity measured by the second measurement wherein the level of the substance being measured is a function of said difference value.
 24. The method of claim 23, wherein the step of determining further comprises comparing the difference value to a table comprising a range of substance levels and a pre-determined difference value corresponding to each level in the range.
 25. The method of claim 23, wherein the step of determining comprises taking a logarithm of the difference value.
 26. A method of measuring the level of a substance in a first solution having a first chemical composition comprising: applying a first electrical voltage to change the pH of a first sample of the first solution, thereby producing a first pH adjusted sample, the first pH adjusted sample having a second chemical composition different from said first chemical composition; performing a first measurement of electromagnetic radiation transmissivity of the first pH adjusted sample; applying a second electrical voltage to change the pH of a second sample of the first solution, thereby producing a second pH adjusted sample, the second pH adjusted sample having a third chemical composition different from said first and second chemical compositions; performing a second measurement of the electromagnetic radiation transmissivity of the second pH adjusted sample; employing said first and second measurements to determine the level of the substance in the first solution.
 27. The method of claim 26, wherein the transmissivity of each of the first pH adjusted sample and the second pH adjusted sample is determined at the same wavelength of electromagnetic radiation.
 28. The method of claim 27, wherein the wavelength is 293 nanometers.
 29. The method of claim 26, wherein the solution comprises pool or spa tub water.
 30. The method of claim 26, wherein the substance is a halogen.
 31. The method of claim 30, wherein the substance is chlorine or bromine.
 32. The method of claim 26, wherein said first measurement and second measurement each comprise a measurement of ultraviolet (UV) light transmissivity.
 33. The method of claim 26, wherein a programmed computer processor employs said first and second measurements as variables to compute the level of said substance in said solution.
 34. The method of claim 26, wherein the step of determining comprises determining a difference value between the light transmissivity measured by the first measurement and the light transmissivity measured by the second measurement wherein the level of the substance being measured is a function of said difference value.
 35. The method of claim 34, wherein the step of determining further comprises comparing the difference value to a table comprising a range of substance levels and a pre-determined difference value corresponding to each level in the range.
 36. The method of claim 26, wherein the step of determining comprises using the result of the first and second measurements in a logarithmic calculation. 